That curve is the result of several things interacting at once: One is that the drag causing it is a force, so it accelerates the bullet's side drift, increasing that drift velocity as the bullet gets further down range. Another is the arc of the trajectory increases as the bullet spends more time slowing and dropping, increasing the upward drag force that causes precession to produce the yaw of repose. Also, the bullet loses spin rate more gradually than it loses forward velocity, so, not counting a temporary trend reversal in the transonic range, the bullet's gyroscopic stability factor increases as the bullet goes down range, which also works to increase the yaw of repose. Both factors cause the spin drift drag to be a larger percentage of total drag, preventing that force from falling off as fast as forward drag does. Finally, as with all drift vectors, as forward velocity falls off, spin drift velocity has time to carry the bullet further during each successive range increment.
I don't think I've ever seen a sight set up to correct spin drift accurately. The problem is the correction changes not only with the bullet and muzzle velocity but with air density as well. It'd take a computerized sight with an air pressure sensor and a way to enter bullet velocity and BC to do it accurately.
I hadn't thought about maintaining sight verticality like that. You got me thinking (always dangerous) that a sight mounted on a ball radial pressure ball bearing slipped over the barrel and that spun around the bore axis, one for the front sight and one for the rear, tied together with a rod underneath to act as your plumb weight so that both front and rear sights stayed upright and at the same height above the bore axis as you canted the rifle left and right would always correct for canting. Of course, there's no guarantee the shooter taking the recoil with the butt plate at a can't wouldn't shift the POI off a little, but over a short range of rotation it would work to keep the sights right.
Hmmm. Actually, that might be made to work. A couple of float arm spirit levels with the peep through the tip of one and the front sight peep through the other. Have to think on how to keep the float angle vertex coincident with the bore line, but it's not impossible.
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